It is common to use a grounding device for rotating shafts in electric motor rotors and other rotary electric machines. Grounding rings or other grounding devices are used to dissipate excess current. If the current is not provided a grounding path, it can arc across rolling element bearings and hasten corrosion, reducing bearing life. In a typical strategy, grounding devices such as grounding rings are used in a fixed housing which prevents rotation of the grounding device. Bristles or other contact elements of the fixed grounding device may then ride on a rotating rotor shaft to provide an electrical ground circuit path for excess current. Electrical current may preferentially flow through the grounding device rather than the bearings, thus preventing or reducing bearing corrosion.
One electrical grounding mechanism is known from U.S. Pat. No. 4,515,417 to Shiraishi. Shiraishi provides a grounding device for preventing electrolytic corrosion in the bearings of a rotary electric machine. The grounding device has a centrifugal contact point between an electrically conductive end cover and an inner surface of a rotor shaft. The contact point includes a spring biased contact element which extends inwardly in the rotor shaft and contacts a portion of the end cover which extends therein. When the rotor is stationary or rotating at low speeds, the contact element maintains electrical contact between the rotor shaft and the end cover. As the rotor speed increases, however, inertia will tend to move the contact element radially outward against the spring bias such that it ceases to provide electrical contact between the rotating rotor shaft and the end cover. A grease lubricated bearing is positioned about and rotatably journals the rotor shaft in Shiraishi. Ostensibly, the electrical resistance of the grease lubricated bearing increases with rotor speed, such that grounding is not needed at relatively higher rotor speeds. Moving the contact element out of contact also purportedly reduces wear. Shiraishi may have application to specific rotor types in specific environments. The design, however, would be poorly suited to different rotor types having relatively lower speed ranges, as well as being poorly suited to designs wherein electrical resistance of rotor bearings does not change sufficiently with increased rotor speed to inhibit electrical currents arcing across the bearings.